1. Field of the Invention
This invention relates to a print hammer apparatus for use in printers which operate with a data processing system and, more particularly, to such hammer apparatus which includes a magnetic flag member which has a flag face that is angularly disposed to the path traversed by the hammer and a stator pole piece that has a pole face that matingly corresponds to the shape of the flag face such that contact bounce is eliminated when the flat face impacts the pole face.
2. Description of the Prior Art
In printers of the type used in data processing equipment, a movable belt supporting a plurality of type characters is moved continuously past a print line at a constant rate of speed. A print medium, such as a web of paper, is fed incrementally, step by step, past the print line, and an aligned array of print hammers is arranged for impact printing along the print line. Because of the relative movement between the type characters on the belt and the print medium during printing, it will be appreciated that the time of printing and, correspondingly, the time of flight taken by the respective print hammers in moving from a cocked position to a striking position and back again must be extremely short to achieve an adequate printing rate, most importantly, must be precisely the same during all strike cycles to assure predictable results. In order to decrease data processing costs, data processing installations are desirous of obtaining printers which exhibit good print quality, require low maintenance, and are lower in cost.
One technique of lowering printer cost is to decrease the cost of the print hammer assembly by reducing the number of individual print hammers, since the hammer assemblies comprise a substantial portion of the total printer cost. Thus, print hammer assemblies which span several type face positions have been fabricated. One example of such a hammer assembly is disclosed in U.S. Pat. No. 3,460,469, entitled "Print Hammer Actuator", and issued on Aug. 12, 1969, in the names of E. A. Brown, R. H. Darling and A. S. Chou. As disclosed therein, the print hammer actuator comprises a magnetic core structure comprising two off-set pole pieces which are formed to define a substantially closed path. A resilient actuator member is mounted with one end fixed relative to the magnetic core structure. A movable magnetic core member is attached to one side of the other end of the actuator member at a position so that the generation of a magnetic flux within the magnetic core structure attracts the movable member, and causes the actuator to be attracted to the pole piece and held thereat in a cocked position. A winding is provided for selective energization to release the movable magnetic member to enable a hammer positioned on the other side of the movable magnetic member to strike a type character. After striking the character the movable magnetic member rebounds off the type and is once again attracted to the pole piece. It will be appreciated that as the resilient member rebounds from the type and moves under the magnetic force of attraction to the pole piece, it acquires a substantial amount of kinetic energy. This kinetic energy causes the movable magnetic member to bounce off the pole piece as it returns to the cocked position.
It must be borne in mind that the hammer bounce problem seriously affects the recycling frequency of the hammer. After a bounce the hammer is positioned some distance from the pole face. Accordingly, before the hammer can be recycled and fired again, it must first be recaptured. It follows that the sum of the flight time and the recapture time determines the recycling frequency of print hammer actuation.
In order to decrease the recycle time, it is necessary to minimize, or eliminate, the time required to recapture the print hammer. Heretofore, attempts to minimize the recapture time have been unsuccessful. First, a visco-elastic damping technique was attempted in which a rubber bumper was secured so as to control the movable member. However, that technique merely increased the initial spring force but did not control recapture. In a second attempt, lubricating oil was applied to the contacting surface of the pole piece and the magnetic member with the hopes that the increase in the viscous forces therebetween would serve to prevent bounce. However, this attempt failed since the dynamic forces at the contacting surfaces was too high. An electrical damping technique was next attempted in which the current in the release winding was reversed at predetermined time. However, the complexity of controlling the required timing of the current reversals in view of the unstable and unpredictable nature of the bounce times, especially when a bank of print hammers was attempted to be controlled, caused this technique to be discarded.
Examples of prior art print hammer assemblies can be found in U.S. Pat. No. 3,349,696, entitled "Hammer Module Assembly in High-speed Printers," which issued Oct. 31, 1967, in the name of John T. Potter; U.S. Pat. No. 3,592,322, entitled "Wire Printing Head," which issued on July 13, 1971, in the names of A. S. Chou and E. A. Brown; and U.S. Pat. No. 3,656,425, entitled "Electromagnetic Actuating Means for Print Hammer," which issued Apr. 18, 1972, in the names of R. T. Albo and J. Pastroni.